When Justin Hansford heard about the death of Michael Brown – actually, when he saw the images of the dead teen’s body – he immediately embarked on the 15-hour drive home to Missouri from Washington, where he was attending a conference.

The assistant professor at the St. Louis University School of Law first saw images of Mike Brown’s death on his social media stream. The unarmed 18-year-old had been shot in Ferguson, a town just northwest of St. Louis, on Aug. 9. His killer was a white cop named Darren Wilson. For Prof. Hansford, who had worked in activist circles well before Mr. Brown’s killing, there was something about the visual aspect of the shooting’s aftermath – the immediacy of seeing exactly what such a shooting looks like...